Transcript Apatite Fission Track (AFT) thermochronology of the Northern Bowser and Sustut Basins: Constraints on the Latest Cretaceous through Cenozoic thermal history Paul O’Sullivan -

Apatite Fission Track (AFT) thermochronology of
the Northern Bowser and Sustut Basins:
Constraints on the Latest Cretaceous through
Cenozoic thermal history
Paul O’Sullivan - Apatite to Zircon, Inc.
Fil Ferri - British Columbia Dept. Energy and Mines
Carol Evenchick - Geological Survey of Canada
Kirk Osadetz - Geological Survey of Canada
Ray Donelick - Apatite to Zircon, Inc.
Project funded by the B.C. Dept. of Energy and Mines,
and the Geological Survey of Canada
Overview of Presentation
- Introduce some general background concerning apatite
fission track (AFT) analyses and interpretation of AFT
data
- Introduce some regional geologic constraints within
the Bowser Basin - these help explain why samples
were originally collected for AFT analysis
- Present representative AFT results from study region
- Summarize the Late Cretaceous to Tertiary thermal
history of the study region based on the AFT results*
*NOTE - I will leave the geologic implications of these results to those
most familiar with the regional geology
Thermochronometers
Temperature (°C)
0
Apatite
100
Apatite
200
K-Spar
300
Zircon
Sphene
Zircon
Topography
Surface processes
Sedimentary Basins
Burial/
Metamorphism
Sphene
400
Muscovite
Metamorphism
Mid-crustal
processes
500
600
700
800
Hornblende
Argon/Argon
Fission Track
U/Th-Helium
Igneous processes
Apatite Fission-Track (AFT)Thermochronology
Constrains maximum paleotemperatures plus timing and style of cooling
from maximum paleotemperatures
Fission-track age
area density of fossil tracks
induced fission tracks (U conc)
“time” of cooling
Fission-track
lengths
‘confined’
lengths
mean track
length
distribution of lengths
“rate” & “style” of cooling
Fission-track kinetics
measure Dpar for each grain analyzed
annealing “resistivity”
1250x
Apatite Fission-Track (AFT)Thermochronology
- Fission-Track Age 220 Ma
Number of tracks provides a
measure of time over which
tracks have accumulated
Pretty young
Bloody old
2000x
Both grains have same
Uranium concentration
Apatite Fission-Track (AFT)Thermochronology
- Fission-Track Lengths “Confined” tracks
Length of a track today is
determined by the maximum
paleotemperatures since that
track formed
Bloody
hot
Pretty
cool
2250x
Apatite Fission-Track (AFT)Thermochronology
- Fission-Track Kinetics Dpar (µm)
“Confined” track
Etch pit
C-axis
Spontaneous
tracks
2000x
Dpar is essentially a measure of apatite solubility in a grain.
Solubility in apatite is related to the grain’s composition
2250x
AFTSolve: Measure kinetic variation (Dpar)
Little variation
between grains
Significant variation
between grains
Total
resetting
~100°C
Dpar is essentially a measure of apatite solubility in a grain.
Solubility in apatite is related to the grain’s composition
Total
resetting
~115°C
~145°C
~160°C
AFTSolve: Modeled Thermal History (not reset)
AFT reset
AFT Age
103±6 Ma
Onset of cooling
88±4 Ma
Potential oil generation
Wet gas
generation
Oil
generation
Stratigraphic Age ~130 Ma
AFTSolve: Modeled Thermal History (reset)
AFT reset
real / predicted
AFT Age = 53±4 Ma / 53±4 Ma
M.L. = 14.3±0.2 µm / 14.2±0.2 µm
Onset of cooling = 56±5 Ma
Wet gas
generation
Oil
generation
Stratigraphic Age ~180 Ma
Lack of control
prior to ~56 Ma
Bowser Basin (Skeena FB)
- regional stratigraphic framework -
Jurassic
Cretaceous
~75 my
meters
3000
~138 my
~144 my
2000
~155 my
~175 my
1000
~200 my
0
Tertiary Plutonic Rocks
Sustut Group
coarsening upward deltaic to
fluvial succession
Bowser Lake Group
coarsening upward
deep marine to deltaic
succession
Hazelton Group
volcanic and
volcaniclastic strata
- Tertiary sedimentary rocks have been eroded from the Skeena Fold Belt
- thus the majority of the post-Cretaceous geologic record has been deleted
- requires regional relationships and/or thermochronology to help
constrain geologic history since ~75 Ma
Bowser Basin (Skeena FB)
1) regional relationships
W
Coast Plutonic
Belt
Intermontane Belt
Skeena Fold Belt
Bowser Basin
Omineca Belt
Sustat
Basin
E
Rocky Mountain
Fold & Thrust Belt
Stikinia
Subducting Farallon/Kula plate
beneath the Coast Complex
North American Craton
- Diving force for deformation within the basin related to subduction of Pacific plates
beneath North American Craton
- Skeena Fold Belt has experienced ~50% horizontal shortening (initially >360 km wide)
At least 2 distinct episodes of folding recognized:
- BLG rocks folded pre-Sustat Basin deposition - older folding <150 Ma and >95 Ma
- Sustat Basin rocks themselves are folded - so younger episode of folding <75 Ma
- Folds in Sustat Basin rocks truncated a by 53 Ma pluton - folds are >53 Ma
Bowser Basin (Skeena FB)
2) regional AFT results
Rapid cooling at
some time between:
~100-80 Ma
~65-60 Ma
~45-35 Ma
~10-8 Ma
W
Coast Plutonic
Belt
??
Intermontane Belt
Skeena Fold Belt
Bowser Basin
Rapid cooling at
some time between:
~65-60 Ma
~45-40 Ma
Omineca Belt
Sustat
Basin
Rapid cooling at
some time between:
~65-60 Ma
~45-40 Ma
E
Rocky Mountain
Fold & Thrust Belt
Stikinia
Subducting Farallon/Kula plate
beneath the Coast Complex
North American Craton
AFT Samples
- locations On GSC Thermal
Maturity Base*
1) Triangle Zone &
Upper Sustut Basin
2) Groundhog coalfield
3) Mount Ritchie
* GSC Report 4343
Evenchick et al. (2002)
Triangle Zone / Upper Sustut
BPF
BLG
BLG
AFT Age
59±3 Ma
Triangle Zone / Upper Sustut
Onset of
Cooling
62±3 Ma
Paleocene
cooling
Not
all
reset
AFT Age
40±4 Ma
Onset of
Cooling
46±4 Ma
AFT Age
40±3 Ma
Onset of
Cooling
44±3 Ma
- Significant Eocene cooling
- Note recent cooling
Groundhog
BLG
BLG
BLG
AFT Age
49±4 Ma
Groundhog
Onset of
Cooling
59±4 Ma
AFT Age
35±2 Ma
Onset of
Cooling
44±4 Ma
AFT Age
45±3 Ma
Onset of
Cooling
48±4 Ma
- Significant Eocene cooling
- Note recent cooling
Mt. Ritchie
BLG
a-3-J
BLG
BLG
AFT Age
38±3 Ma
Mt. Ritchie
Onset of
Cooling
41±3 Ma
AFT Age
41±5 Ma
Onset of
Cooling
a-3-J
43±4 Ma
AFT Age
46±5 Ma
Onset of
Cooling
49±5 Ma
- Significant Eocene cooling
- Note recent cooling
Mt. Ritchie
No known tectonic mechanism to justify higher paleogeothermal gradients thus we prefer the constant gradient solution (A)
Timing
Northern Bowser
Constant Values:
~65-60 Ma
~45-30 Ma
~10-5 Ma
Western Bowser
Bowser Basin (Skeena FB)
- regional AFT results Rapid cooling at
some time between:
~100-80 Ma
~65-60 Ma
~45-35 Ma
~10-8 Ma
W
Coast Plutonic
Belt
Rapid cooling
sometime:
~65-60 Ma
~45-30 Ma
~10-5 Ma
Intermontane Belt
Skeena Fold Belt
Bowser Basin
Rapid cooling at
some time between:
~65-60 Ma
~45-40 Ma
??
Omineca Belt
Sustat
Basin
Rapid cooling at
some time between:
~65-60 Ma
~45-40 Ma
??
E
Rocky Mountain
Fold & Thrust Belt
Stikinia
Subducting Farallon/Kula plate
beneath the Coast Complex
North American Craton
So what do AFT results suggest?
- AFT results are interpreted to record rapid cooling from paleotemperatures
≥110-160°C (range based on different annealing resistivities from Dpar)
- Cooling is interpreted to have occurred in association with km-scale
denudation throughout the region during the Paleocene, Eocene, Miocene
Remember:
- At least 2 distinct episodes of folding within the Skeena Fold Belt have been
recognized:
1) >95 Ma, but <150 Ma - BLG rocks folded prior to Sustat Basin deposition
2) >53 Ma, but <75 Ma - Sustat Basin rocks folded, trucated by 53 Ma pluton
- Major episodes of denudation occurred within the Skeena Fold Belt after the
last known phase of contractional deformation (between 75-53 Ma)
- Results do not discount the possibility that minor reactivation within the fold
belt might have occurred more recently than previously documented
Furthermore:
- Incorporating regional thermal maturity data (%VR) with the AFT
results and paleogeothermal gradient information derived from
the a-3-J well, suggests that at least 7.0 to 4.4 km of
sedimentary section has been removed over sections of the
Bowser Basin (assuming geothermal gradients of 31°C/km and
45°C/km respectively)
- Hydrocarbon generation from potential source beds ceased with
rapid cooling/denudation sometime between latest Cretaceous
and Paleocene times
- Petroleum generation ceased earlier in overmature regions
Bowser Basin / Skeena Fold Belt
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
Fission Track Methodology
- System Response 1 -
2
3
*TA = Total Annealing (resetting)
1
3
2
Small Dpar TA*
Large Dpar TA*
Number of tracks
1
Temperature (°C)
Time (Ma)
Final distribution
(at time 3)
Track length (µm)
2
3
*TA = Total Annealing (resetting)
Number of tracks
1
Temperature (°C)
Fission Track Methodology
- System Response 2 Time (Ma)
1
3
Small Dpar TA*
2
Large Dpar TA*
Final distribution
(at time 3)
Track length (µm)
Bowser Basin / Skeena Fold Belt
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
Lunch?
Time for a
Canadian, eh?
Quick Time™ an d a
TIFF ( Un compr ess ed ) de co mpr es sor
ar e n eed ed to s ee this pic tur e.
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.